1. Field of the Invention
The present invention relates to a method for removing sulfur oxides from a stack gas.
2. Description of the Prior Art
Absorption processes for removing sulfur oxide from a stack gas are known where a slurry comprising a calcium compound is used, e.g., CaO, CaCO.sub.3, Ca(OH).sub.2, etc., as the absorbent. Such process generally comprise two stages:
A. AN ABSORPTION STAGE, WHERE THE SULFUR OXIDE IS ABSORBED FROM THE STACK GAS AND CONVERTED TO THE SULFITE FORM; AND
B. AN OXIDATION STAGE WHERE THE ABSORBED SULFUR OXIDE IN THE SULFITE FORM IS OXIDIZED TO GYPSUM.
In the absorbing stage, CaSO.sub.3 is produced by reaction according to formulae (1) or (2), EQU Ca(OH).sub.2 + SO.sub.2 .fwdarw.CaSO.sub.3 + H.sub.2 O (1) EQU CaCO.sub.3 + SO.sub.2 .fwdarw. CaSO.sub.3 + CO.sub.2 ( 2)
the solubility of CaCO.sub.3 in water is so small that a "blinding" phenomenon, i.e., a covering of particles of CA(OH).sub.2 or CaCO.sub.3 which have not yet reacted, occurs.
This blinding phenomenon is the greatest demerit in desulfurizing a stack gas using a calcium compound, causing a drop in the SO.sub.2 absorbing efficiency or in the utilization efficiency of Ca.
To alleviate the above fault, slurry wet shredders or delay tanks in the circulatory system of the absorbing slurry of the calcium compound have been used. However, even such devices do not enable a utilization efficiency of the calcium compound of 100% to be obtained, and a 20 to 100% surplus of Ca(OH).sub.2 or CaCO.sub.3 must be supplied.
Thus, following the prior art substantial absorption efficiency problems are invariably encountered in the absorption step. Further, prior art processes also suffer from defects in the oxidation step.
The object of the oxidation step is to recover good quality gypsum from the CaSO.sub.3 resulting from the sulfur oxide absorption. For the formation of good quality gypsum, the pH of the absorbent slurry must be kept under 5 during oxidation.
Usually, sulfuric acid is added to regulate the pH of the absorbent slurry, but most of the sulfuric acid is consumed in neutralizing the Ca(OH).sub.2 or CaCO.sub.3 present, and in this case the before-mentioned blinding phenomenon again occurs.
Furthermore, dropping the pH of the circulating absorbent slurry to decrease scaling and to increase the utilization of Ca(OH).sub.2 and CaCO.sub.3 to keep the desulfurizing rate high requires severe conditions.